Spark plugs, particularly those in high-speed, high-compression engines, are subjected to an extreme range of pressure and temperature conditions. Plug temperatures range from about 200.degree. C. at low engine speeds and light loads, to as high as 850.degree. C. under full throttle, full load. Below about 450.degree. C., carbon and other products of combustion begin to form on the plug insulator nose. If not removed, those deposits build up until current shorts through the deposits instead of sparking across the electrodes. At normal speeds, enough heat is usually generated to burn those deposits away as quickly as they are formed. However, when high speeds or heavy loads raise the plug temperatures above 600.degree. C. to 700.degree. C., deposits that have not been burned away, particularly those resulting from the additives in currently available fuels and lubricants, are melted to form a glaze coating on the plug insulator nose. When hot, this glaze is highly conductive and the plug is shorted out. This causes misfiring with consequent fuel and power losses. Should plug temperatures become excessive, the plug points themselves become hot enough to ignite the fuel-air mixture in the cylinder. This causes auto-ignition and, if continued, can lead to the destruction of the plug and serious engine damage. Overheated electrodes also cause a condition commonly met in two-stroke engines: the bridging of the electrodes due to the build-up of conducting deposits formed by combustion particles which have melted upon their striking the overheated electrodes. In plug temperatures ranges above 850.degree. C., chemical corrosion and spark erosion cause plug failure within a very short time.
It will be seen then, if a hot-type plug is subjected to high compression Pressures, temperatures, and loads, electrode burning and auto-ignition will result because of the plug's slow rate of heat transfer. A cold plug, because it will not reach full operating temperature, will not tolerate low-speed, light-load operation for any length of time without becoming fouled with current-conducting deposits. Because a cold plug under such conditions will not reach a temperature required to burn off fouling, carbon formation as well as additive particles from the fuel and oil will condense on the comparatively cool surfaces of the insulator to foul the plug and to cause it to misfire.
Spark plugs are customarily supplied in various heat ranges to handle the requirements of individual engines and operating conditions. Heat range refers to the ability of the plug to conduct the heat of combustion away from the electrodes or firing end. A conventional hot-type plug will have a long insulator nose. Because of the length of the heat path, heat thus will be transferred comparatively slowly from the plug firing end to the engine cooling system. A conventional cold-type plug, on the other hand, has a comparatively short insulator nose and heat is transferred rapidly into the engine's cooling system.
Therefore, to overcome the foregoing and other difficulties of the prior art, the general object of this invention is to provide means for varying the heat range of a spark plug automatically to thus keep the plug at the most effective temperature during all operating conditions such that starting, warm-up, idling, low- and high-speed operation of the engine are improved. And, further, to accompany such improvement in engine performance with an efficient spark plug design that reduces the causes of misfiring so that the engine produces greater power and increased fuel economy in all speed ranges.
It is another object of this invention to provide a multiple heat range spark plug whose operating temperature is automatically varied such that the plug runs hot at the lower cylinder temperatures occurring when the engine is idling or at low speeds and loads to thereby inhibit plug fouling, and which runs relatively cool at higher cylinder temperatures such as those occurring under conditions of high speeds and loads so as to prevent the plug overheating that causes auto-ignition and plug electrode burning.
Another object is to provide a spark plug whose design eliminates the requirement for a specific heat range in a plug so that the number of spark plug types required to be manufactured or that have to be stocked by the dealer are thereby reduced. A concomitant object is to provide a spark plug having a multiple heat range such that the selection of a plug with the proper heat range for a specific engine or for the type or service that the engine will encounter will no longer be a problem such that the possibility of fitting plugs of the wrong heat range in an engine with the attendant probability of poor performance and engine damage or owner dissatisfaction is thereby avoided.
Yet another object is to provide a spark plug having automatic means for varying the heat range such that an optimum operating temperature is maintained to thereby minimize the plug fouling that leads to the misfiring which results in engine emissions that contribute heavily to environmental air pollution. In addition, it is an object to provide a plug that will maintain a high standard of performance with engine fuels that have their volatility reduced and have some of their additives and compounds eliminated as a pollution curb.